The present invention relates to locking threaded fasteners, and more particularly to lock nuts that include a coil or a band as a one-way locking element.
Lock nuts can include a coil or a band as a one-way locking element. Typically, the locking element has one end bent radially outwardly and anchored to the nut, typically nested in a close fitting slot in the nut. Under loosening torque, this anchoring method results in a tension load in the locking element that causes the element to tighten its grip on the external thread to prevent relative rotation (loosening) between the nut and external thread. The opposite force, compression, is exerted on the locking element during installation. This force causes the locking element to loosen its grip on the external thread so that there is relatively low installation torque.
This locking system typically require special means to enable removal of the nut from the external thread. However, at least one lock nut does not require special means as disclosed in U.S. application Ser. No. 12/880,580 filed Sep. 13, 2010. That lock nut includes a locking element that slips at a controlled removal torque, simplifying the construction of the nut by eliminating special release rings.
The described lock nuts have not gained widespread adoption perhaps because either the complexity, and therefore cost, is too high or the performance is not comparable to competitive technologies. One performance challenge is related to the manner in which locking elements have been anchored to the nuts. The radial tang experiences high reaction loads when removal torque is applied to the nut. Even if the locking element is designed with a strong cross-section, such as a rectangular wire band as disclosed in the '580 application, there can be high stress on the external thread adjacent the anchored tang. This high stress causes excessive wear on the external thread, especially if the locking element is stainless steel, and increases friction between the band and the external thread. The friction can cause the slip torque to rise above the strength limit of the band, resulting in fracture or permanent set of the band. This prevents reuse of the nut.